The present invention relates to a joining system head for attachment to a movable frame, in particular to a robot, having                A holding means for an element to be joined to a part, and        A joining drive means to move the holding means along a joining direction for joining.        A feeding means for feeding elements to the joining system head.The present invention relates further to a joining system having a robot movable on at least two coordinate axes and a joining system head attached to the robot. Lastly, the present invention relates to a method of feeding elements from a stationary unit to a movable joining system head and joining said elements to parts by means of the joining system head.        
Such a joining system head, such a joining system and such a method of feeding and joining elements by means of a joining system head are generally known. The term ‘joining’ in the present context is intended to refer to all ways of connecting elements to parts, in particular connections of metal elements to metal parts, for example by bonding, forming, as for example riveting, or by union of matter, as for example welding, including short-time arc welding. Short-time arc welding is often referred to as bolt welding, even though it is not exclusively bolts that are welded. A current system of bolt welding in industrial use, in combination with a robot, is known in the brochure “Neue TUCKER Technologie. Bolzenschweiβen mit System!,” Emhart TUCKER, Sep. 1999.
Bolt welding finds application chiefly, but not exclusively, in vehicular technology. Here, metal elements such as metal bolts, with or without threads, eyes, nuts etc., are welded onto the sheet metal of the bodywork. The metal elements then serve as anchors, or fastening elements, to fix for example interior fittings, lines and the like to the sheet metal of the body. At the joining system head, disclosed in the above-mentioned Emhart TUCKER publication, the joining drive means is configured either as a linear electric motor or as a combination of a lift magnet and a spring.
The holding means is constituted by a one-piece tongs elastically expandable in radial direction. The elements are as a rule welding bolts comprising a head having a somewhat larger diameter than the shank of the bolt. In the known system, the bolts are fed to the welding head by way of suitable feeding conduits by means of compressed air. The bolts are thus fed ‘head first’ into the tongs from behind. Ordinarily the bolt will strike the tongs from the inside, but without passing through it. A loading pin provided coaxial with the tongs is then actuated to propel the bolt thus fed through the tongs. The tongs are elastically expanded radially when the head of the bolt passes through. Then the tongs snap closed elastically around the shank of the bolt and hold it fast in the position determined by the travel of the pin.
The joining drive means in the form of a linear motor (or lift magnet/spring combination) has a travel of a few millimeters. Also, the welding head is fixed at the end of an arm of the robot, usually by way of a pneumatic or hydraulic carriage. That is, the entire welding head is movable in a direction parallel to the welding axis by means of the carriage, which has a considerably greater travel than the linear motor. The welding head further comprises a control means to control the linear motor and the loading pin, provided spatially separate from the welding head, more specifically in a stationary feeder.
To perform a welding operation, first the robot is programmed so that it travels into a predetermined position in which the carriage and linear motor axes are perpendicular to the sheet metal onto which the bolt is to be welded. The bolt is prestressed so that it protrudes vis-á-vis a supporting foot. Then the carriage is actuated until the foot meets the sheet metal. The bolt held in the holding means then rests in contact with the sheet metal. Next comes a determination of the zero line of the holding means with respect to the sheet metal. Alternatively, however, there are methods of zero line determination that dispense with the supporting foot.
Then, in the case of welding with supporting foot, an electric pre-current is switched on, passing through the bolt and the part. The bolt is then lifted relative to the part by means of the linear motor (lifting means). An electric arc is set up. Then a switch is made to the welding current. By the high welding current, the opposed faces of bolt and part begin to be fused. The bolt is then lowered onto the part again, so that the respective melts will mingle. Upon attainment of the part and the short circuit of the arc, or just before, the welding current is switched off. The entire melt solidifies and the welded connection is complete.
Now the welding head is drawn off from the welded-on bolt, using the carriage. The carriage is necessary because, among other reasons, the drawing-off motion must take place exactly on the centerline of the welded-on bolt. Otherwise, owing to the one-piece tongs, there would be danger of damage to the bolt and/or the tongs. The robot arm alone is not capable of such a precise linear motion in an arbitrary direction of space. For owing to the superposition of the simultaneous regulation of several components of robot arm motion, as required for this purpose, such linear motions can be executed by the robot with a certain amount of undulation only. The known welding head comprises a comparatively great axial extent. Since moreover the welding head must be drawn off from the bolt in an axial direction, use of the welding head in places of difficult access is possible only within limits.
Then there are developments for employing robot technology to feed the bolt. Here a separate pick-up takes pre-sorted bolts and brings them to the welding location. This is disclosed in “Bolzenschweiβen. Grundlagen und Anwendung” by Trillmich, Welz, Fachbuchreihe Schweiβtechnik, DVS Verlag, 1997, Chapter 9.3. It is there explained that this technology lends itself especially to headed bolts that, because of their size and shape, cannot be blown through hoses. This type is referred to as the “pick-up system.”
Further, a welding head by the firm of Nelson has been disclosed, in which a lift device moves a carrier projecting laterally arm-like up and down. At the terminal portion of the carrier, a holding means with tongs is rigidly mounted. The bolts are fed, as in the case of the TUCKER welding head described above, to the tongs from behind, by means of a compressed air hose extending through the carrier. The end portion of the carrier with holding device fixed thereto is more readily positioned at inaccessible locations. The lift device to move the projecting arm and the pertinent control means are arranged in the initial portion of the carrier.
Against this background, the object of the invention consists in specifying an improved joining system head, an improved joining system and an improved method of feeding and joining fed elements. This object is accomplished, for the joining system head initially mentioned, in that a control means to control the joining drive means is arranged at the joining system head spatially separate from the holding means and the joining drive means, so that the holding means and the joining drive means form a joining tool of small dimensions, and in that means are provided to pass on the elements fed to a transfer station of the feeding means to the holding means in front in each instance.
The joining system head according to the invention represents a completely novel concept. This is based on two fundamental ideas. One of these ideas consists in providing the control means at the joining system head, but spatially distanced from the holding means and the joining drive means. The holding means and the joining drive means can consequently form a joining tool of small dimensions and not much interference edge relevance. The other idea consists in feeding the elements by means of the feeding means, not immediately all the way to the holding means, but as far as a transfer station likewise distanced from the joining tool. In addition, means are provided to pass on each of the elements fed to the transfer station to the holding means from in front. In this way it is possible to keep the over-all axial length of the joining tool small, since no loading pin is required to position the elements through the holding means from behind.
Since the joining drive means and the holding means are integrated into a joining tool, no transmission of a lifting motion over long distances (cantilever arm or the like) is needed. Consequently the positioning and the actual joining or welding can take place locally with high precision. In the joining system according to the invention, the above object is accomplished in that a joining system head according to the invention is fixed to the robot arm.
The method according to the invention for feeding elements from a stationary unit to a movable joining system head and joining fed elements to parts by means of the joining system head contains the steps of feeding an element from the stationary unit to the movable joining system head while the joining system head is joining an already fed element to a part. According to the invention, the elements are not conveyed from the stationary unit all the way to the holding means in one step as in the prior art. Instead, the elements are at first fed from the stationary unit only as far as the transfer means. This feeding step can accordingly take place while the joining welding head itself is joining an already fed element to a part. By virtue of this partial processing, shorter cycle times are obtainable over all. The object has thus been completely accomplished.
It is of especial advantage if the joining tool is mounted on an end portion of a projecting elongated carrier. The spatial distance between joining tool and control means is consequently attained by the elongated carrier. This makes it possible to bring the joining tool to inaccessible locations through openings.
Here it is especially preferred if the joining tool is movably mounted on the end portion of the carrier and if a loading drive means is designed to move the joining tool to the transfer station in order to pass an element on to the holding means. In this embodiment, it is especially advantageous that the movable mounting of the joining tool creates an additional degree of freedom for positioning the same. For example, the joining tool, when it has already been introduced into a cavity by means of the carrier, can still be moved. This permits flexible “handling.” Further, the movable mounting of the joining tool makes it possible for the means of passing an element on from the transfer station to be realized by the mobility of the joining tool. The joining tool consequently in each instance itself ‘fetches’ the elements to be joined from the transfer station.
Further, it is preferred in this embodiment if the joining tool is rotatably mounted on the end portion of the carrier and if the loading drive means is designed to rotate the joining tool as far as the transfer station. A rotatability of the joining tool on an end portion of the carrier is comparatively simple to achieve as a matter of design. Rotatability as a single degree of freedom is sufficient for numerous applications. When it is considered that the carrier itself is also rotatable about its lengthwise axis by means of the robot as a rule, and arbitrarily positionable in space, joining operations can be carried out even in highly inaccessible locations. The interference edge profile of the joining tool is determined by the necessary radius of swing.
The loading drive means serves generally, in the first place, to fetch one element at a time from the transfer station, and in the second place, also to position the joining tool relative to the carrier in a welding position. It will be understood that the variability will be greater the greater the angular range within which welding positions can be arranged by the loading drive means.
According to an alternative embodiment, the loading drive means comprises a motor arranged at the end portion of the carrier. In this embodiment, a precise control of the joining tool can be achieved with good response behavior.
In an alternative embodiment, the loading drive means comprises a motor arranged in the neighborhood of the control means and a transmission that transmits motions of the motor to the joining tool. In this embodiment, an improved interference edge clearance results, since the interference-edge relevant end portion of the carrier does not comprise any motor of its own to move the joining tool. Rather, the comparatively bulky motor is arranged in the neighborhood of the control means and transmits its motion to the joining tool by way of a transmission.
Here it is especially preferred if the motor is a rotary motor, in particular an electric motor, and if the transmission is a transmission with tension means. With a rotary motor, motions can be executed precisely and with high responsiveness. With the tension transmission, comparatively great distances between the neighborhood of the control means on the one hand and the end portion of the carrier on the other hand can be achieved by comparatively simple design.
Upon the whole, it is of advantage if the joining drive means consists of a linear electric motor. In this embodiment, the joining drive means is configured as a lift means. A linear electric motor requires only relatively few lines to trigger it, and can be regulated in both lift directions.
Here it is of especial advantage if the longitudinal axis of the joining drive means and the longitudinal axis of the holding means are distanced parallel to each other. In this embodiment, it is possible so to position the holding means that even welding positions close to edges are attainable. The distance between the longitudinal axes may be in the range of some few centimeters, just enough to shift the holding means out of the projection of the joining drive means in joining direction.
According to a further preferred embodiment, the holding means comprises a plurality of jaws arranged distributed around the longitudinal axis of the holding means and movable towards and away from each other, in order to hold or release an element, according to the case. Here it is especially preferred if the holding means comprises two jaws. The term ‘jaws’ in the present context is to be understood broadly. The jaws may for example be elongated fingers. With two fingers, rotationally symmetrical or approximately rotationally symmetrical parts in particular can be grasped with comparative convenience and held securely.
It is preferred if the jaws are movable far enough away from each other so that the holding means can release the element by being drawn off from the element oblique to the joining direction. This embodiment makes it possible to perform the operation of ‘withdrawing’ the joining system head from the element joined to the part by means of the robot alone. A carriage for guiding a completely rectilinear return motion is then not required. In that sense, this embodiment also contributes to a smaller axial extent of the welding head.
It is especially preferred, however, if the jaws are movable far enough away from each other so that the holding means can release the element by being swung away from the element about an axis of rotation oriented transverse to the joining direction. In this embodiment, the jaws can be moved away from each other far enough so that the joining tool need not be withdrawn in joining direction. Rather, it is possible after the joining operation to withdraw the joining tool transverse, in particular perpendicular, to the joining direction, the element being passed between the jaws of the holding means. In this embodiment, consequently, no axial motion is required.
In this way it is possible to pass the carrier with joining tool arranged on the anterior end portion through even extremely small openings, and execute joining operations inside of cavities. The carrier, after attaining the joining position, can remain positioned almost without change. After the joining operation, the joining tool is moved, in particular swung, away transverse to the joining direction, and then the carrier can be withdrawn from the cavity again along its longitudinal axis.
Further, this embodiment makes it possible for the elements to be grasped in especially simple manner by the transfer station. The joining tool is so moved, in particular swung, in this embodiment, that the holding means with released jaws is aligned with an element at the transfer station. Then the element is grasped by the jaws, and taken out of the transfer station by a motion, in particular a swinging motion.
In general, it is preferred if a jaw actuator is provided, actively opening and/or closing the jaws. In this embodiment, as a rule the jaws are configured as rigid fingers. The jaw actuator ensures that the jaws are either actively opened, to release an element, or else actively closed, to hold the element.
Alternatively to this, it is possible to configure or mount the jaws elastically, so that they are passively movable towards and/or away from each other. In this embodiment, the jaws may either be made of an elastic material, in which case other elastic means are superfluous as a rule, or else the jaws may be configured as rigid elements and elastically mounted. It is also possible, within the scope of this embodiment, for the jaws to be elastically pre-stressed, either in holding or in releasing direction. In that case, as a rule an actuator is provided, which actively moves the jaws in the respective other direction.
In general, in an especially preferred embodiment, the transfer station is arranged on the longitudinal carrier. In this way it is possible to achieve a fixed relative position of the transfer station with respect to the joining tool. Besides, it is advantageous for the cross section of the carrier to be smaller as a rule than the cross section of the joining tool, so that space will be available for the transfer station.
Further, it is generally of advantage if the feeding means comprises a magazine for elements. In this way, it is possible always to place an element in readiness to be ‘grasped’ at the transfer station.
In the joining system according to the invention it is of advantage if a stationary individuating means conveys individual elements all the way to the transfer station of the joining welding head. This embodiment serves in general to enhance the degree of automation. Such stationary individuating and feeding means are known per se in the prior art. However, they convey individuated elements all the way to the holding means in one step, whereas in the joining system according to the invention, a conveyance takes place only as far as the feeding means (transfer station). Thence the holding means ‘fetches’ an element conveyed thither. It will be understood that the features mentioned above and those yet to be illustrated may be employed not only in the combination specified in each instance but also in other combinations or by themselves, without departing from the scope of the present invention.